1. Field of the Invention
The present invention generally relates to pixel clock creation and phase control widely used in image forming apparatuses including laser printers and digital copiers. More particularly, the present invention relates to a pixel clock creation method and device which realize highly precise phase control of the pixel clock, an optical scanning device and an image forming apparatus equipped with the pixel clock creation device.
2. Description of the Related Art
FIG. 43 shows the composition of the image forming apparatus, such as laser printer, digital copier etc.
As shown in FIG. 43, the scanning of the laser light which is emitted from the semiconductor laser 501 is carried out by the polygon mirror 502 which is rotated. The laser light from the polygon mirror 502 is focused through the scanning lens 503 and forms the optical spot on the photoconductor 504 which is the scanned medium. The photoconductor 504 is exposed to the laser light so that the electrostatic latent image is formed.
At this time, the photodetection unit 505 detects the scanning light for every scanning line, and the phase lock circuit 509 creates the image clock (pixel clock) with the phase synchronized for every scanning line, based on the clock of the clock creation circuit 508 and the output signal of the photodetector 505. The phase lock circuit 509 supplies the pixel clock to each of the image-processing unit 506 and the laser drive circuit 507.
The image-processing unit 506 creates the image data on the basis of the pixel clock supplied from the phase lock circuit 509, and outputs the image data to the laser drive circuit 507.
The laser drive circuit 507 controls the emission time of the semiconductor laser 501 in accordance with the image clock with the phase locked by the phase lock circuit 509 for every scanning line and in accordance with the image data created by the image-processing unit 506.
In the above-mentioned scanning optical system, the variations of the distance from the rotation axis of the deflection reflection surface of the deflector, such as the polygon scanner may occur, and the variations may cause the irregularities of the scanning speed of the optical spot (the scanning beam) which optically scans the surface of the photoconductor.
The scanning-speed irregularities may cause fluctuations of the reproduced image, and the image quality will be degraded. Thus, if a high-quality image is demanded, it is necessary to correct such scanning irregularities.
Furthermore, in the case of the multi-beam optical system, when there is a difference in the oscillation wavelength of each of the light sources and the chromatic aberration of the scanning lens is not corrected in the optical system, the deviation of the exposure position occurs. The difference of the scanning width arises for each of the light sources when the spot corresponding to each source of luminescence scans the surface of the scanned medium, and such difference may cause the degradation of image quality. To avoid this, it is necessary to correct the scannng width.
The conventional technology for correcting the scanning irregularities is, for example, the method of changing the frequency of the image clock based on the predetermined characteristics of the scanning optical system, so that the optical spot position along the scanning line is controlled (for example, see Japanese Laid-Open Patent Application No. 11-167081 and Japanese Laid-Open Patent Application No. 2001-228415).
Moreover, by detecting the first horizontal sync signal and the second horizontal sync signal corresponding to the starting point terminal point, and carrying out calculation of the number of the clocks for every line, with two photodetectors installed in the both ends of the photoconductor as other technology, it asks for scanning speed and the method of controlling the rotational speed of the polygon mirror by the motor is learned.
It becomes complicated constituting of the image clock control unit the conventional technology (this being called frequency modulation method) of changing the frequency of the image clock.
Moreover, this complexity increases as frequency modulation width becomes minute.
For this reason, there is the problem that fine control cannot be performed, by the conventional frequency modulation method.
Moreover, by the method of controlling the rotary motor of the deflector like the polygon mirror, the limit is in the control accuracy. That is, as a factor which generates scanning irregularities, the rotation jitter of the deflector, expansion and contraction of the scanning lens by temperature change, etc. exist.
Therefore, even if it is the optical beam deflected by the same deviation reflection surface, it is difficult to cancel scanning irregularities, and by the method of controlling the rotary motor of the deflector, the limit is in the control accuracy.